The invention relates to dual cure coating compositions and to a process for the production of multilayer coatings using the dual cure coating compositions, in particular for the production of outer clear coats or top coats of multilayer coatings.
In relation to the present invention, the term xe2x80x9cdual cure coating compositionxe2x80x9d means a coating composition that is curable by free-radical polymerization on UV irradiation and additionally by thermally induced polyaddition and/or polycondensation with the participation of hydroxyl groups.
U.S. Pat. No. 4,212,901, for example, discloses dual cure coating compositions based on polyester polyols esterified with (meth)acrylic acid which are curable by free-radical polymerization of olefinic double bonds on UV irradiation and additional polyisocyanate curing agents.
In the case of three-dimensional substrates of complicated geometry, for example automotive bodies, it has not hitherto been satisfactorily possible to achieve an acceptable combined level of technological properties, for example, hardness, solvent resistance, acid resistance, scratch and mar resistance (particularly when the automobile is subjected to automatic car washing) on all the different surface zones of a substrate with a coating layer applied from a prior art dual cure coating composition. xe2x80x9cDifferent surface zonesxe2x80x9d means zones of the surface accessible to irradiation with UV radiation and shaded areas (zones of the surface accessible to irradiation with UV radiation with difficulty or only in part or inaccessible zones). While the technological properties of the dual cure coating layer are excellent in surface zones directly accessible to UV radiation due to the combination of thermal and UV curing, the level of properties in the shaded areas which have only been cured thermally is poor, in particular hardness is inadequate in these areas.
Increasing the hydroxyl value of the hydroxy-functional binder comprising olefinic double bonds in a dual cure coating composition with the aim of improving the technological properties of the coating layer in shaded areas generally results, due to the accompanying increase in the binder""s olefinic double bond equivalent weight (mass in grams which contains one mole of olefinic double bonds) which conventionally occurs, in a degradation of the level of properties of the coating layer in surface zones which are cured both by UV irradiation and thermally. Conversely, when the olefinic double bond equivalent weight is reduced, the accompanying reduction in hydroxyl value which conventionally occurs results in a degradation of the level of properties in shaded areas. On the other hand, the aim is not straightforwardly achieved by simply increasing the hydroxyl value and simultaneously reducing the olefinic double bond equivalent weight because compatibility problems frequently occur between such binders and other constituents of the coating composition.
There was a requirement to find dual cure coating compositions which do not exhibit the known weaknesses; coating layers applied there from should exhibit an at least satisfactory level of properties when only thermally cured and excellent properties when cured both thermally and by UV irradiation.
The present invention meets this requirement by providing a dual cure coating composition which achieves the stated object, which composition contains a reaction product of a specific polyester polyol with an isocyanate-functional component comprising olefinic double bonds, wherein said reaction product is distinguished by good compatibility with other binders, reactive diluents, solvents as well as cross-linking agents.
The present invention provides a dual cure coating composition, the resin solids content of which is composed of
(a) 30 to 90, preferably 40 to 70 wt. % of at least one hydroxy-functional polyester urethane having olefinic double bonds,
(b) 0 to 60, preferably 5 to 50 wt. % of at least one binder other than the polyester urethane (a) and/or at least one reactive diluent, and
(c) 10 to 50 wt. % of at least one cross-linking agent for hydroxy-functional components (a) and optionally (b),
wherein the at least one polyester urethane (a) has a number average molecular mass of 1000 to 5000 and a hydroxyl value of 80 to 250 mg KOH/g, contains olefinic double bonds corresponding to an olefinic double bond equivalent weight of 250 to 2000 and is obtainable by the reaction, which proceeds with consumption of all isocyanate groups, of a component (a1) containing at least one isocyanate group and at least one olefinic double bond with a non-aromatic polyester polyol (a2) having a calculated molecular mass of 600 to 1400, preferably of 800 to 1200, an acid value of 0 to 30 mg KOH/g and a hydroxyl value of 250 to 600, preferably of 270 to 400 mg KOH/g at a calculated hydroxyl functionality of 4.5 to 10, preferably of 4.8 to 8, and wherein the sum of weight percentages of components (a) to (c) is 100 wt. %.